Objective: This study was carried out in order to ascertain whether or not Hurricane Katrina and related factors (i.e. the amount of rainfall) influenced the male-to-female birth ratio (M/F). Materials and Methods: Monthly births by gender for the affected states (Alabama, Florida, Louisiana and Mississippi) for January 2003 to December 2012 were obtained from the Centers for Disease Control and Prevention (CDC Wonder, Atlanta, Ga., USA). Precipitation data was obtained from the US National Weather Service. Ordinary linear logistic regression was used for trend analysis. A p value ≤0.05 was taken to represent a statistically significant result. Results: Of the total of 3,903,660 live births, 1,996,966 (51.16%) were male and 1,906,694 (48.84%) were female. Significant seasonal variation was noted (the maximum M/F in May was 1.055, the minimum M/F in September was 1.041, p = 0.0073). There was also a separate and significant rise in M/F 8-10 months after the storm (April to June 2006, peak M/F 1.078, p = 0.0074), which translated to an approximate deficit of 800 girls compared to 46,072 girls born in that period if the M/F increase was theoretically only due to a girls' deficit in the denominator of the ratio. This spike was only present in Alabama, Louisiana and Mississippi, all of which received heavy rainfall. Florida did not receive heavy rainfall and experienced no such M/F spike. Conclusion: In this study there was a dose-response relation between the amount of rainfall after Hurricane Katrina and the monthly sex ratio of live births in the US states of Alabama, Louisiana and Mississippi 8-10 months later. The well-known yet unexplained distinct sex ratio seasonality may be due to natural or man-made radiation contained in the rain.